The present invention relates to a magnetic read/write device which writes data to a recording medium as magnetic information by use of a write head and then reads and decodes the written magnetic information by use of a read head. More particularly, the present invention relates to a technique of preventing malfunction due to a reversal in the polarity of the read output from the read head.
One method for increasing the recording density of a magnetic read/write device is to use a GMR (Giant Magnetoresistive) head, which has become very common. A GMR head includes a free layer whose magnetization direction can be changed by an external magnetic field. Since the resistance of the GMR element changes with the magnetization direction of the free layer, written information (on the disk) can be read by converting this change in the resistance into voltages.
Even though the polarity of this (voltage) output usually does not change, a polarity reversal may possibly occur while the head is operating, depending on the configuration of the head. If this happens, the waveform of the read signal will become inverted. Conventional GMR heads are designed such that no output polarity reversal occurs. However, as the device size and the distance between the read shields have been reduced, it has become difficult to ensure sufficient resistance to polarity reversal. To provide a reduced head device size, the “automatic pinning type” GMR head has been studied and developed in which an antiferromagnetic layer is not disposed adjacent the pinned layer. The “automatic pinning type” GMR head has very good read performance but is disadvantageous in that the magnetization direction of the pinned layer tends to be readily reversed due to contact with the medium, etc.
If a reversal in the polarity of the read output has occurred, the magnetic read/write device may become unable to decode read positional information, or may incorrectly decode it, which prevents the magnetic read/write device from positioning the head. Under the worst case conditions, the magnetic read/write device might run out of control and break down. Even if such a runaway does not occur, the magnetic read/write device will become unable to position the head, leading to an inability to decode user data.
To accommodate reversal of the output polarity due to degradation of the read head, Patent Document 1 (Japanese Patent Laid-Open No. 2000-251203) discloses a method which includes the steps of: when a read data error has occurred, reading a reference signal (or an address mark) from the servo data and determining the polarity of the output; and upon detecting a polarity reversal, supplying a refresh current to the read head to return the output polarity to normal.
In the case of the “pre-servo write” system, to accommodate reversal of the output polarity due to the difference between each head in the magnetic read/write device and each head (in the servo writer) used to perform servo write operation to the device, Patent Document 2 (Japanese Patent Laid-Open No. 2003-85704) discloses a method which stores the output polarity of each head and thereby allows the magnetic read/write device to read servo information even when there is a polarity difference between these heads by utilizing the polarity switching function of the read channel.